Distortion correction in transmission systems



w. P. MAsoN Filed Oct. 3, 1928 F/G. l

Dec. 15, 1931.

DIsToRTIoN'coRREcTIoN 1N TRANSMISSION SYSTEMS W E MA .s0/v @y A TTRNEY Dec. 15, 1931. w. P. MASON DISTORTION CORRECTION IN TRANSMISSION SYSTEMS Filed Oct. 5, 1928 2 Sheets--Sheet NEGAT/VE' /MPDANCE A NEGATIVE /MPEDANCE Dfi/ICE nAl A vvv V 5i/:lli-

DEV/CE k- 25A /NVE/VTR W. E MAso/v ATTURNEY Patented Dec. 15, 19.31

UNITED STATI-is PA'rEmOFFIOE.N

WARREN PERRY MASON, OF EAST ORANGE, NEW JERSEY,'ASSIG1IOR To BELL TELE- PHONE LABORATORIES, INCORPORATED, OAF NEW YORK,

NEW YORK N. Y., .e CORPORATION OF DISTORTION cORREcTiON 1N TRANSMISSION SYSTEMS Application filed October 3, 1928. Serial No. 310,166. j

A This invention'relates tO systems for transmitting electrical Wave energy, such as telephone Vtransmission systems and the like, and particularly to correction of distortionin such systems.

l? rom an ideal quality standpoint a transmission system should be designed so that the currentsl received thereover are exact copies of the corresponding currents entering the system at the transmitting end; in other words, the transmission systemv should be distortionless. Then electrical Waves,.such as telephonie currents, are transmitted over conductors of relatively great length, the distor tion produced in the Waves due to the inherent properties of the conductors becomes of increasingimportance. ln long high quality systems,` suchV as are now being used for telephone` transmission, program broadcasting, television, etc. the distortion produced by l ines is often of sach magnitude as to impair appreciably the quality ofthe received Waves, and thus reduces the commercial efficiency of such systems. Distortion in transmission lines is, in genof three different kinds-first, attenuation of the transmitted Waves by the line; second, variation in the degree of attenuation of Waves of diiferent frequencies by the line; and third, a relative phase change in the Atransmitted Waves of diiferent frequencies. rl`he cause of such distortion is the improper proportioning of the resistance, the leakance, the capacity and the inductance of the line.

The Pupin-Campbe1l and other methods of loading transmission systems have been devised with the obj ect of reducing attenuation in the useful frequency range in such systems. The use of such loading methods in transmission systems has been supplemented in practice .by theuse of amplifiers, such as .the tclephonerepeater, to. compensate for reduction in the amplitude of the transmitted Waves. The second type of distortion inherent in transmission systems, an unequal decree of attenuation in the transmitted Waves of different frequencies, may be Overcome by the use of attenuation equalizing networks at different points inthe system, this expedient having been brought to. ahigh stateof development in practice. Phase corrective networks or phase compensators to supplement other types of distortion correction, such as loading, have been used in connection with lines to correct phase distortion in the transmitted Waves overa desired range Of frequencies. Other networks have been devised and utilized in connection with transmission lines to Vcombine the functions Of phase distortion correction and attenuation equalization in transmitted Waves of a desired range of frequencies. The latter net- Works have been Veither designed, in part at least, by. empirical methods, or else, in their design all factors contributing to distortion have not been taken into account. Their use, therefore, enables onlyan approximation of the ideal distortionless line to be Obtained. Y

An object of the invention is to correct distortion in electric Wave transmission.

Another and more specific object is to reduce the attenuation in, and to minimize variations inthe` velocity. of propagation of electric Waves over transmission systems.

A feature of the invention is a unitary net- Work Which, when combined with a transmission line, without altering the characteristic impedance 4of theline, Will simultane-V ouslyV compensate for amplitude and phase distortion in Waves of all frequencies transmitted thereover, and at'the same time Will restore the Waves vto their Original amplitudes.

VIn accordance with the invention, exact design formulae have been developed for the impedance relations which aV structure must satisfy When combined With a transmission line in a given manner in order to completely compensate for distortion produced bythe line in Waves of all frequencies transmitted thereover. Networks of different types have been designed in accordance with these formulae, which, when combined With a transmission system or line in a given mannerv Will, Without altering the characteristic impedance of the line, reduce or substantially compensate for theamplitude and phasevdistortion in Waves of allfrequencies transmitted thereover.

loo

More sp ecifically st ated the above mentioned objects and features have been obtained in ac.- cordance with the invention by inserting in a transmission system or line in the path of waves transmitted thereover an impedance structure having one or more series arms and one or more lattice or shunt arms, the structure as a whole having a characteristic impedance equal to that of said line, and a propagation const-ant, the negative of that of said line.

In a preferred embodiment of the ,invention, the compensating structure is of the lattice type. Each of the two .series arms of this lattice structure comprises a short cir- -cuited section of smooth line half as long as, and with the same characteristic impedance as the line to be compensated before the compensating structure was inserted therein, in tandem with a device, which will effectively change the positive impedance of the short circuited line to a negative impedance of equal value. Each of the two lattice arms of the lattice network comprises an open-circuited section of smooth line half as long as, and with the same characteristic impedance as the line to be conpensated before the insei-tion of the compensating structure therein, in tandem with a device which will effectively change the positive impedance of the open circuited line to a negative impedance of equal value.

In anotherrembodimentV of the invention the compensating structure comprises a T type structure. Each of the two series arms of this T structure comprises a short circuited section of smooth line half as long` as the line to be compensated, and having the same characteristic impedance as the line to be compensated before insertion therein of the compensating structure, and in tandem therewith a device which will effectively change its positive impedance to a negative impedance of equal value. The shunt arm of the T net.- work comprises an open-circuited section of smooth line, the same length as, and having the same characteristic impedance as the line to be compensated in series withv an impedance which is the equivalent of that of the series arm above described, and in tandem therewith a device which will change the net impedance of the whole combination to a negative impedance of equal value. An exten- 'sion of this embodiment is the vr network in which Fig. 1 is the diagram of a transmission system employing idistortion correcting networks of the lattice type embodying the invention;

Figs. 2 and 3 show diagrammatically the construction of the lattice networks used in the system of Fie. l;

Fig. 4 shows diagrammatically a modification of the invention requiring fewer impedances inthe distortion correcting networks than employed in the system of Fig. l;

Fig. 5 shows diagrammatically a modification of the invention employing T type networks for the distortion correcting structures;

Figs. 6 and 7 show diagrammatically the construction of the impedance arms of the T type networks of Fig. 5;

Fig. 8 shows diagrammatically a modification of the invention employing vr type networks for the distortion correcting structures;

Figs. 9 and lO show diagrammatically the construction of the impedance arms in the 1r type networks of Fig. 6; and

Fig. ll shows a type of simulating network which may be used in place of each line in the structures of Figs. 2, 3, 6, 7, 9 and l() to approximate the results obtainable with the artificial lines of said structures.

In Fifi'. l is shown a transmission line for transmitting alternating current waves of a plurality of frequencies generated by a source 2 to a load circuit represented by the resistance 3. The line I comprises a plurality of smooth line sections S1, Sg, which are indicated in the figure byV their equivalent T networks having two series arms, each of impedance v and a shunt arm of impedance sinh g where ZZ is the characteristic impedance of line l and T is the propagation coeiicient for a section of line l of length Z.

Inserted in line l between each two adjacent sections is a el-terminal network II which is designed in accordance with the inventionto correct for the attenuation distortion and phase distortion in transmitted waves of all frequencies produced by a section of line l of length Z, for example, a section comprising the adjacent half of section S1 and the adjacent half of section S2. Each network N is of the lattice type comprising no series impedance arms represented by the boxes Z1 and two lattice impedance arms .represented by the boxes Z2.

. The values for the impedances of Z1 and ZL are determined in the following manner.

The propagation coellicient P and the characteristic impedance ZK of the complex line including the smooth sections S1, S2, and

iJU

the distortion correcting networks N are given by the following equations:

cosh P=cosh cosh F14-sinh? sinh I1 2ZZ 2ZLZ1, coth F1 ZF1 coth E+ Zlz tanhE ZKaSZIzx n v 2V 2 2 Z1, 1/Z1Z2 cosh F1 (3) Let l Z1= Z1 taub v(4) and l i I z2 2, 00u15 V(5) Substituting in Equation it is found that Z11=Zz `Fr: T 6) Substituting the values for Z1, and Il in Equations (1) and (2), it is found coshP=1 or P==O (7) and ZK is indeterminate, and in fact equals the terminating impedance.

Hence, if the impedances in the arms Z1 and Z2 of the networks N in the system are given the values of Equations (4) and (5), the complex line will produce zero overall attenuation and a theoretically infinite or very high, constant velocity inthe transmitted waves of all frequencies.

Equation (4) for the series impedance Z1 of networks N may be physically realized by the structure of Fig. 2. The structure of Fig. 2, which in the system of Fig. 1 would be inserted in the boxes Zlin series with the terminals thereof comprises a Short-circuited section of smooth artificial line 4 half as long as, and with the same characteristic impedance as the section of line 1 in which the distortion isto be compensated, and in tandem therewith a device 5 which will effectively change the positive impedance of artificial line 4 to a negative impedance of equal value.

The negative impedance device 5 ma be of any suitable type, for example', the eedback amplifier described and claimed in the patent to R. Mathes, No. 1,779,382, is

sued October 21, 1930, whichwill effectively give between two terminals thereof for a wide range of frequencies, Athe negative of a resistance, inductance, capacity or complex combination thereof utilized as the feedback impedance. lf the structure of Fig. 1 Vof lthe Mathes patent is used as the. device 5 in the structure of Fig. 2 of this application, the terminals ZY1 of t ev latter structure would be connected to the terminals 16 and 17 ofA the former structure, and one pair of terminals of network 4 of said latter structure would be Vconnected to the terminals of box '14Vin the circuit of said former structure so Y that network 4 acts as the feed-back impede ance Z in said circuit.

Vrlhe short-circuitedlength of line 4 has been represented in Fig. 2 by its equivalent T network having two series arms each of impedance and a shunt arm of impedance sinh g Equation (5) for the lattice'impedance Z2 of the networks N may be physically realized by the structure of Fig-3. This structure, which in the system of Fig. 1 would be inserted in the boxes Z2 in series with the terminals thereof, comprises an open-circuited section of smooth line 6 half as long as, and with the same characteristic impedance as the section of line 1 to be compensated, and in tandem therewith a device 7, similar to the device 5 of Fig. 2, which will effectively change the positive impedance of line 6 to a negative impedance of equal value.

The section of line 6 vhas been represented i in Fig. 4 by its equivalent T network having two series arms each of impedance In- Fig. 4 is shown a. modification of the system of Fig. 1 in which only one impedtransmitted waves of all frequencies pro# duced by a section of line l. This is accomplished by using two transformers in con-` junction with the correction impedance ele-.

One transformer 8, 8

ments Zl and Z2. effectively connects the structure Z1, which is equivalent to the structure Z1 in the system of F ig. l, in series with the line l, and another transformer comprising two parts 9, 9 effectively couples the impedance structure Z2, which is equivalent to the structure Z2 of the system of Fig. l, respectively in series with the lattice arms l() and l1 of the network. An inductance of value M1, where M1 is the mutual impedance of transformer 8, 8', is connected in series with the impedance structure Z1 and the secondary windings of transformer 8, 8. An inductance of value M2, where M2 is the mutual impedance of the lattice transformer 9, 9 is connected in series with the impedance structure Z2 and the secondary windings` of transformer 9, 9.

A lattice network N', such as shown in Fig. 4, with equal effective series impedances Z1 and a mutual impedance M1 between them, and two lattice arms with equal impedances VZ2 and a mutual impedance M2 between them is equivalent to a lattice network with equal .series arms of impedances Z1 lll 1, and equal lattice arms of impedances Z2-M 2, with no mutual impedances appearing in the circuit. If the two primary windings of transformer 8, 8 in series with line 1 have an equal number of turns and each is coupled to a secondary winding which has the same number of turns as each of the primary windings, then each series impedance Z1 of the lattice net- Awork will be equal 'to the teri'ninating impedance ZT of transformer 8, 8 minus the mutual impedance M1, that is, f

Then, if the terminating` impedance of the series transformer 8, 8 is made equal to Z1 tdlh `2+M1,

the impedance of each series arm reduces to Z1= Z1 magi-M-MF Z1 mag 9) Similarly, if the primary windings of the transformer 9, 9 in the lattice arms lO and 1l of the lattic network N have an equal number of turns and each is coupled to a secondary winding having the same number of turns as the primary windings, each lattice impedance will be equal to the terminating impedance ZT1 of thetra-nsformer 9, 9 minus the mutual impedance M2, that is,

Zz-"Za-M2 10) .2

Then, if the terminating impedance of transformer 9, 9 is made equalto Z, coth g-l-M2;

the impedance of each lattice arm reduces to Z2: Zl 00th gJfMVMF-Zwothg (u) vThe series impedances Z1: Z1 tanhg and the lattice impedance Z2: Z1 00th g) may be physically realized by the structures of Figs. 2 and 3, respectively, which have .been described.

In F ig. 5 is shown a transmission system equialent to that of F ig. l except-that a T type struct-ure 'l' having two series arms of impedance Z1, represented by the boxes so labeled, and a shunt arm of impedance Z1, represented by a boxY so labeled, is inserted between each two adjacent line sections S1, S2, in place of the lattice type structure N of Fig. l, the impedance arms of this T structure being designed to compensate completely for the phase and amplitude distortion in transmitted waves of all frequencies produced by a section of line l of length Z.

The smooth sections S1, S2, S3 in line l may be represented, as indicated in Fig. 5, by their equivalent T networks comprising two series arms each of impedance and a shunt arm of impedance I" sinh 2 Tt is apparent, therefore, that if the impedances Z2 in the series arms of network T each have an impedance of and the impedance Z.1 in the shunt arm of network T an impedance of Z1 sinh 1" Y 2 the-effective series and shunt impedances of one section of line l of length Z will be effectively cancelled. Therefore, cach T structure will effectively compensate the distortion in both phase and amplitude produced by one section o f line l oflength Z in the transmitted ,waves forall frequencies.

The impedanceZ3 in each series arm .of the structures Tin the systemfv Fig. 5 will have a value of Y if the elements thereof connected in series with its terminals are as shown in Fig. 6.- The structure of Fig. 6 comprisesa shortcircuited length of smooth line 12 half as long as, and with the same characteristic impedance as the section of line "1 in which the distortion is to be compensated, and in tandem therewith a device 13, similar to the device 5 of Fig. 2, which will effectively change the positive impedance ofnetwork 12 to a negative impedance of equal value.'

The line 12 in Fig. 6 has been represented by its equivalent T network comprising two series arms each of impedance `Z1 taub.

if-the'elements connectedvin serieswithiits terminals are as-sho'wnin Fig. 7 Thestruc:v

y 1 to be compensated, in vseriesv with-a structure 15 comprising a short-circuited length of line 16 half as long as, and with the same characteristic impedance Vas .the section of line to be compensated, in tandem with a device 17, similar to the device 5 of Fig. 2, which will effectively change the positiveimpedance of line 16 to a negative impedance ofequal value. and in tandem with thev combination a device V18, similar to theldevice 17, which will effectively change `the positive impedance of the combination to. anegative impedance of equalvalue. y. i The' section of articial line 14;y in Fig. 'l' has been represented by its equivalent T network comprising twoseries arms each ofimpedance f and a shunt arm of impedance i" The section of artificial line16 of Fig.

been rrepresented byits 'equivalent T network having two series arms each of impedance anda shunt arm if impedance sinh g It is apparent, therefore, that if the structure Z5 in the series arm of each 1r correcting network has an impedance of Zlf sinh I, and the structure vZ6 in each shunt arm of the 7.- structure has an impedance of tion of v.line will be eiiectively canceled by each qr network and the overall phase and amplitude distortion in-.the system may beV made zero. 5

The shunt impedance structure Z will have it the structureV of Fig. 9 is connected in series with its terminals. The structure of Fig. 9 comprises an open-circuited section of smooth line 19 half as long as, and with the same characteristic impedance as, the section vof line 1 to be compensated, and in tandem therewith a device 20, similar to the device 5 of Fig. 2, which will effectively change the positive impedance of line 19 to a negative impedance of equal value. The open-circuited length of line 20 in Fio'; 9 has been represented by its equivalent T network having two series arms each of impedance Zl 0 sinh-g *Y have been represented in the figure i The'impedance Z5 in the v'series .arm of each .1r network inthe system of Fig. f8 will fhave an impedance of -Zl sinh I if elements such as shown in the structure of Fig. 10 are connected in series between its terminals. The structure of Fig. 10 comprises a parallel-combination of a section of short-circuited line 2l of the same length as, -and with the same characteristic impedance as the section of line 1 to be compensated, and a section Yof open-circuitedline 22 half as long as, and with lthe same characteristic impedance as the lsection of line l to be compensated in tandem with a device 23 which will change the positive impedance of line 22 to a negative impedance of equal value, and in tandem with the parallel combination a device 24, similar to device 23, which will change the positive vimpedance of the combination to a negativ-e impedance ot' equal value.

.The short-circuited length of line 22 has .been represent-ed in Fig. l0 by its equivalent vr network comprising la series arm :of impedance Z1 sinh F `and two shunt arms each of impedance i and .the open-circuitedline 22 by its yequina- .lent T .network comprising two series arms each of impedance Y and a .shunt arm .of impedance A simulating network, such as shoWnin Fig. l1, may be used in place of one or more of the sections of smooth line in the structures of Figs. 2, '3, 6, 7, 9 and 10 to obtain an approximation of the results, as regards phase and amplitude distortion correction, .obtainable with `these structures when the sections of smooth line are employed therein. The simulating network of Fig. ll is a. T structure having two series arms each comprising a resistance of value in series with an inductance'of l and a shunt arm comprising a parallel combination of a resistance of value and a capacity of value C, where R is the resistance, L the inductances, G the leakance,

and Cthe capacity of the section of line l to be compensated.

It should be noted that each impedance element in the structures of the invention which have been illustrated and described contributes :both to attenuation distortion correction and phase distortion correction thisbeing Aan important distinguishing teature from thestructures of the prior art in `.which w'attenuation distortion correction or phase distortion correction is the sole function of separate elements therein.

Alth'oughthe invention as described has been principally directed to the obtaining of complete compensation for attenuation and phase'distortion, it is apparent that it may be utilized also for obtaining any desired degree 'of attenuation yand*v phase distortion correction.

Although the invention has been described in connection with a particular transmission system comprising a plurality ot uniform smooth sections .of line anda yplurality of distortion correcting networks spaced at equal intervals along the transmission line, the invention is not so limited :but is applicable also to a transmission system comprising a smooth 0r a loaded line in combination with a single distortion Acorrecting network or a plurality of such networks inserted at unequal intervals along the line, orany other combination within the scope of the :appended claims.

What is claimed is:

A1. In combination in a wave transmission system, atransmission line, and means for substantially neutralizingphase and amplitudedistortion yin waves of all frequencies transmitted thereover, said means comprising a complex electrical structure comprising a plurality of impedance arms inserted in said line the'path of said waves, and having propagation and impedance characteristics substantially complemental to those of said Aline, each of said impedance arms comprising a section of smooth line in tandem with a device which will change its positive impedance to va negative impedance of equal value.

"2. A combination in accordance with claim l and ink which said impedance arms of said complex electrical structure comprise an impedance arm effectively in series with said transmission line and an impedance arm efectivelyrin shunt with said transmission line.

3. In combination in a wave transmission system, a transmission line, and means for completely neutralizing amplitude and phase distortion in waves of all frequencies transmitted thereover, saidmeans comprising an electrical structure inserted in said line in the pathof said waves, and having a plurality of impedance arms, said structure having propagation characteristics which at all frequencies are effectively the negative of the propagation characteristics oi' said transmission line, fat least one of said impedance arms including anopen circuited smooth line'half the length of said transmission line and hav.` ing the same characteristicimpedance assaid transmission line beforethe insertion Vof said structure therein. 4 l

el.V In combination in a system, a transmission line, and means for completely7 neutralizing amplitude and phase distortion in waves of all frequencies transniitted thereover, said means comprising an electrical structure inserted in said line in the path of said waves, and having a plurality of impedance arms, said structure having a propagation characteristic which is the negative of that of said line, and a characteristic impedance equal to that of said line, at least one of said impedance arms including an open circuited line half the length of said transmission line and having the same characteri, tic impedance as said transmission-line before the insertion of said structure therein, and in tandem therewith a device which will change the positive impedance of said open circuited line to a negative impedance of equal value.

5. In combination in a wave transmission system, a transmission line, and means for over, said means comprising an electrical structure inserted in said linein the path of said waves,'and having a plurality of impedance arms, said structure having propagation characteristics which are atV all frequencies 'effectively the negative of those of said transmission line, at least one of said impedance arms including a short circuited smooth line half the length of said transmission line and having the same characteristic impedance as said transmission line before insertion of said structure therein.

6. In combination in a Wave transmission system, a transmission line, and means for neutralizing amplitude and phase distortion in Waves of all frequencies transmitted thereover, said means comprising an electrical structure inserted in said line in the path of said waves, and having a plurality of impedance arms, said structure having at all frequencies propagation characteristics which are effectively the negative of those of said transmission line, at least one of said impedance arms including a short circuited smooth line half the length of said transmission line and having the same characteristic impedance as said transmissionline before the insertion of said structure therein, and in tandem therewith a device which will change the positive impedance of said short circuited line to a negative impedance of equal value.

7. In combination with a wave transmission system, a transmission line, and'means for substantially neutralizing amplitude and phase distortion in electrical waves of all frequencies transmitted thereover, said means wave transmission transmitted waves of all frequencies, saidV means compris-ing a bridge network of the lattice type inserted in said line in the path of said Waves, and comprising two series impedance arms and two lattice impedance arms, each of said series arms having a characterist-ic impedance Z, mnh

and each of said lattice arms having a characteristic impedance V vZz Y l where Z l is the characteristic impedance and I the propagation constant for said transmission line before the insertion of said compensating means therein. l Y

V9.l In combination in a Wavetransmission system, a transmission line for transmitting electrical Waves of different frequencies and a bridge network of the lattice type inserted in. said line in theV path of said waves and servingjto completely correct phase and amplitude distortion produced by said line on waves of all frequencies transmitted thereover, said network comprising two series arms and two lattice arms, each of said series arms comprising a short circuited smooth line half the length of said transmission line and having the same characteristic impedance as said transmission line without said bridge network and in tandem therewith a device which will change the positive impedance of said short circuiited line to a negative impedance of equall value, and each of said lattice arms compri-sing an open circuited line half the length of said transmission line and having the 'same characteristic impedance as said transmission line without said bridge network and in tandem therewith device which. will effectively change the positive impedance of sai-dopen circuited line to a negative impedance of equal value.

l0. ln combination. a line for tran-smitting'waves of a plurality of frequencies and means forcompensating for the phase and amplitude distortion produced bysaid line on transmitted waves of all frequencies, said i comnrisin-g a bridge network of the lattice type inserted in said line in the path of said waves, said networkcomprising two arms of equal impedance-in series with said line and two -rms of-equal impedance in lattice relation thereto, a series y ransformer, a

lattice transformer, a circuit comprising an impedance substantially equal to and an inductance substantially equal to M1 in series therewith, connected eiifectively in series with each of said seriesarms by said series transformer, and a second circuit comprising an impedance substantially equal to and an inductance of value lt 2 in series therewith, connected effectively in series with each of said lattice arms by said lattice transformer, where ZZ is the characteristic impedance and I the propagation constant for said line before the insertion of said compensating means therein, M1 is the mutual impedance of said series transformer and M2 is the mutual impedance of said lattice transformer.

l1. The combination of claim 10 and in which said impedance substantially equal to '-Z, mtg s comprises a short circuited section of smooth line half the length of, and with the same characteristic impedance as said transmission line to be compensated without said bridge network and in tandem therewith a device which will effectively change a positive impedance of said short circuited section of line to a negative impedance of equal value.

12. The combination of claim 10 and in which said impedance substantially equal to comprises an open circuited section of smooth line halt the length of, and with the same characteristic impedance as said transmission line to be compensated without said bridge network, and in tandem with a device which will eectively change the positive impedance of said open circuited section of line to a negative impedance of equal value.

13. An electrical fnetwork comprising a plurality of impedance elements, a single negative impedance device, and means for coupling said single negative impedance device simultaneously with two or more of said impedance elements so as to produce between the terminals of each of the coupled impedance elements what is effectively a negative impedancelet. An electrical network comprising a number of impedance elements at least two of which have substantially equivalent impedance characteristics, and a single negative impedance device simultaneously coupled to and cooperating with said equivalent impedance elements to produce between the 

